The present invention relates to a spindle motor having a rotor rotatably supported on a stator through a liquid dynamic pressure bearing having a flanged shaft member and a cylindrical receiving member in which the shaft member is rotatably received and having a cup-formed hub press-fitted on the flanged shaft member, and to a method for assembling the spindle motor.
The present assignee applicant has developed and applied for a patent (Japanese Patent Application No. H09-36380/1997) for a liquid dynamic pressure bearing having a flanged shaft member 1 and a cylindrical receiving member 4, one embodiment of which is shown in FIG. 7, and a spindle motor having such a liquid dynamic pressure bearing. That is, the liquid dynamic pressure bearing shown in FIG. 7 has a flanged shaft member 1 in a cross-formed section integrally formed by a cylindrical member 2 and a thrust dynamic pressure disk member 3, and a cylindrical receiving member 4. An annular lid member 5 serves also as a thrust holding plate.
The spindle motor includes, besides the liquid dynamic pressure bearing structured as above, a cup-formed hub 6 coaxially fixed on an end of the flanged shaft member 1 to support a rotary member such as a hard disk, a rotor magnet 7 attached on an inner peripheral surface of a sleeve portion of the cup-formed hub 6, a stator coil 8 attached on an outer peripheral surface of the cylindrical receiving member 4 to generate a rotation force in cooperation with the rotor magnet 7, and a base plate 9 in which the cylindrical receiving member 4 is provided.
The liquid dynamic pressure bearing, structured by the flanged shaft member 1 having a cross-formed section formed with a thrust dynamic pressure disk member 3a at an axial center and the cylindrical receiving member 4 rotatably accommodating the flanged shaft member, is removed of a problem involved in the conventional dynamic pressure bearing structured by a flanged shaft member having a T-formed section formed with a thrust dynamic pressure disk member 3a at one end of the shaft and a receiving member rotatably accommodating the flanged shaft member, i.e., rotation instability resulting from half whale phenomenon caused by disagreement in direction of a shaft displacement and a restoration force against the displacement during high speed rotation.
However, there has been a problem that the flanged shaft member 1 having the cross-formed section integrally formed with two members of the cylindrical member 2 and the thrust dynamic pressure disk member 3a is fabricated by machining and hence is high in manufacture cost. To avoid this, a spindle motor one embodiment of which is as shown in FIG. 6 has been developed. That is, in FIG. 6 a flanged shaft member 1 is fabricated by machining as respective separate members one cylindrical member 2 and a dynamic pressure producing ring member 3 and then assembling these two members together. This solved the problem with manufacture cost involved in the spindle motor of FIG. 7.
Nevertheless, another problem has arisen in assembling such a spindle motor. The assembling of the spindle motor of FIG. 6 includes, after a process of inserting and arranging in a cylindrical receiving member 4 a flanged shaft member 1 fabricated by press-fitting a dynamic pressure ring member 3 onto a cylindrical member 2, a process of press-fitting an annular holding member 5 in an annular step 4b formed in an opening end of the cylindrical receiving member 4, and thereafter a process of press-fitting a cup-formed hub 6 having a mount hole 6a at a center onto the flanged shaft member 1. The problem lies in that the dynamic pressure ring member 3 deviates off a specified press-fit position when press-fitting the hub 6 onto the flanged shaft member 1 by applying an urging force. This deviation is caused due to receiving an urging force applied to the hub 6 at the annular step 4a at a boundary of smaller and larger diameter cylindrical portions of the cylindrical receiving member 4 when press-fitting the hub 6 to the flanged shaft member 1 by applying an urging force. This deviation is slight as small as on the order of a micron, but crucial for a liquid dynamic pressure bearing.
The above problem with assembling would not occur if a cylindrical member 2 and a cup-formed hub 6 were made in one body. However, the integral form with a cylindrical member 2 and a cup-formed hub 6 is extremely difficult to mass-produce at low cost, similarly to a flanged shaft member 1 having a cross-formed section integrally formed with two members of a cylindrical member 2 and a dynamic pressure ring member 3.
Eventually, for a spindle motor having a liquid dynamic pressure bearing having as basic constituent members a flanged shaft member fabricated by press-fitting a dynamic pressure ring member on a cylindrical member and a cylindrical receiving member rotatably receiving this shaft member, it is left unsolved to press-fit the cup-formed hub onto the flanged shaft member in a manner not to deviate the dynamic pressure ring member off a specified press-fit position. Due to this, it is impossible to meet the requirement of mass-producing the spindle motors at low cost.